In response to the recent jump in crude oil prices and green energy policies, energy policies for reducing fossil fuels have been published and enforced. As for Korea with 97% of national total energy coming from imports, an annual average energy consumption increase rate reaches 1.1% and fossil fuels account for 83% of the total energy source, and Korea has generated more carbon dioxide than the advanced countries, and, thus, along with the strengthening of the greenhouse gas emission regulations according to the convention on climate change and international environmental regulations, its industrial competitiveness has been weakened. Under these environmental regulations and energy policies, eco-friendly electric vehicles and smart grids have received a lot of attention, and, thus, energy storage devices need to be developed and are growing rapidly.
A secondary battery is a key component in constituting an energy storage device. The secondary device refers to a battery configured to convert electric energy into chemical energy to be stored and then convert the stored chemical energy into electric energy to be used if necessary, and includes an electrode material, a battery/capacitor, a module/pack/battery management system, and the like. Such secondary devices may include lithium ion batteries, lithium ion polymer batteries, metal air batteries, redox flow batteries, sodium sulfur batteries, magnesium ion batteries, sodium ion batteries, nickel hydrogen batteries, NiCd batteries, and the like, and technologies regarding parts thereof and materials and components of supercapacitors also belong to the secondary battery technologies. The secondary batteries can be classified depending on the purpose of application, into small-scale energy storage systems such as mobile technologies; medium-scale energy storage systems such as electric vehicles and home lithium battery cells/modules; and large-scale energy storage systems such as large-sized batteries.
The capacitor refers to a device, i.e., a storage battery, configured to store electricity. Particularly, a supercapacitor refers to an ultra-high capacitance capacitor with a very high electric capacitance, and is also referred to as an electrochemical capacitor and uses charging caused by simple movements of ions to an interface between an electrode and an electrolyte or a surface chemical reaction unlike a battery using a chemical reaction. Therefore, the supercapacitor can be charged and discharged at a high speed and has a high charge/discharge efficiency and a semipermanent cycle life and thus has been used as an auxiliary battery or battery substitution, and with the recent remarkable increase in new renewable energy, the supercapacitor has received a lot of attention as a principal energy storage device. Particularly, in relation to securing energy, the supercapacitor may be used for power generation using wind power, sunlight, and fuel cells, and, thus, it is possible to stably obtain electric energy and supply high-quality electric power.
An electric double-layer capacitor (EDLC) using activated carbon as an electrode and electric double-layer charge adsorption as a mechanism is configured to store electric energy by physical adsorption, and, thus, the EDLC does not have a life reduction problem caused by charging/discharging unlike secondary batteries and thus has an advantage in terms of maintenance and has received a lot of attention due to its merits such as high-speed charging and high power. However, the EDLC has a disadvantage of being much smaller than a lithium ion battery with an energy storage capacity of 100 Wh/kg or more.
Lithium ion batteries have high energy density and thus have been supplied as power sources for mobile phones, PC, and digital cameras, and their use has been expanded to power sources for hybrid car or electric vehicles, but some prerequisites such as safety and cycle characteristics still remain. Accordingly, a hybrid supercapacitor as a capacitor capable of being charged and discharged at a high speed with a high energy density needs to be developed, and studies for application to various fields are being conducted.
Further, when a supercapacitor is produced, activated carbon for forming an electrode accounts for 43% of the material cost, which means that the electrode is the dominant component that determines the characteristics and price of the supercapacitor, and, thus, a high efficiency and economic feasibility need to be considered.
In addition to activated carbon which has been typically used as a carbonaceous electrode material, there are various alternative materials. Specifically, examples thereof may include graphene, carbon onion, carbon nanotube, carbide-induced carbon, and templated carbon. Particularly, graphene has excellent physical and electrical properties and is a noticeable new material. However, in order to show its excellent properties, graphene needs to be exfoliated to atom layer thickness, and such mechanical exfoliation has a low yield. Therefore, currently, a method of obtaining reduced graphene by preparing graphene oxide and then reducing the graphene oxide via a chemical process is the most commonly used. However, the reduction method using a high-temperature reducing gas is not suitable for mass production and increases the unit cost of production.
A high energy density along with excellent power density and robust cycle life of electrode structures during repeated ion insertion/desertion reactions are critical to satisfy the more challenging standards in performance for future electrochemical energy storage systems such as hybrid plug-in electric vehicles (HPEVs) and even pure electric vehicles (EVs). Currently, the dominating electrochemical energy storage remains on a lithium ion battery (LIB) with high energy density although an electrochemical capacitor (EC) with high power density along with robust cycle life has great potential for many energy storage devices. This is because the EC has the relatively low specific energy density compared to that of the LIB. In addition, it was found that the sole usage of an LIB or an EC alone could not provide simultaneously high energy and power densities because of its complementary ion storage mechanism. In this view, a lithium ion hybrid capacitor (LHC) has been recently suggested as one of the promising energy storages in that the LHC could take the advantages of battery and capacitor energy storage mechanisms on conjugation of the battery-type anode along with the capacitor-type cathode. However, there exist other challenges to realize a high-performance LHC due to several obstacles such as kinetic imbalance and poor capacity in the full-cell configuration of anode and cathode electrode materials. Meanwhile, the conversion or alloying reaction in the anode electrode for the LHC was found to be better for a high capacity. The Sn metal capable of leading to an alloying reaction is one of the great candidates to realize the anode electrode for an LHC since the Sn metal's high theoretical specific capacity of 990 mAhg−1 and operation potential of ˜0.25 V vs Li/Li+ have a potential to give a higher energy density required for the next-generation energy storage, while the Sn metal's large volume expansion, typically reaching 300% during an alloying reaction, may cause pulverization of Sn metals and interruption of electron/ion transportation. This in turn leads to fast fading of capacity, thus resulting in short cycle life and poor rate capability. One approach to overcome these drawbacks is to reduce the average size of crystalline particles for the electrode to the scale of several nanometers as ultrafine nanoparticles (NPs) can mitigate the strain induced by a large volume change of particles and retard pulverization. The problem is in that even ultrafine metal NPs on a simple substrate can be agglomerated during repeated redox cycles, thus eventually causing capacity fading during repeated discharge/charge cycles.
Korean Patent No. 10-0866311 discloses a method for preparing a nitrogen-rich nanoporous graphite carbon nitride structure.